38 DEVELOPMENT OP CEREBRO-SPINAL SPACES IN PIG AND IN MAN. 



in the pig; but these play little part until the chorioid plexuses become of sufficient 

 size to divide the ventricle into a superior and inferior portion. In the human 

 embryo, as in the pig, the superior half of the ventricular roof is sacrificed to the 

 greater growth of the cerebellum. 



In human embryos of 17 mm., however, these factors have not begun to influ- 

 ence the membranous area. This is shown in figures 58 and 59, photomicrographs 

 from embryo No. 576. The section is somewhat to the side of the midline, but in 

 the superior portion of the roof of the fourth ventricle the differentiated membra- 

 nous area can be made out. The sharp delimitation of this area from the denser 

 typical ependyma on both sides is quite apparent. The ragged character of the 

 ventricular border, with its few elongated spindles, seems wholly in keeping with the 

 transverse view of this area afforded by figure 37. 



Embryo No. 576 exhibits one characteristic of the area membranacea superior 

 very frequently seen in human embryos, but almost invariably absent in these stages 

 in the pig. Along the lateral margins of the superior membranous area are dense 

 borders of the many-layered epithelial-like cells which lined the ventricular roof in 

 younger stages. This feature is well shown in figures 60 and 61, the latter figure 

 being a higher magnification of the former. The cellular border of the superior area 

 reaches transversely only through a few 15-micron sections, but it extends throughout 

 the whole cephalo-caudal diameter of the area. It seems likely that this represents 

 purely a survival of the epithelial-like cells in the younger embryos. In rarer 

 instances the whole area membranacea superior may be surrounded by such a border 

 of many-layered cells, but even in these cases the superior and inferior margins 

 are quite thin. 



No apparent agencies favoring the disappearance of the superior membranous 

 area in the roof of the fourth ventricle of the human embryo are apparent in stages 

 up to the fetus. Thus, in human embryos of 18 mm. this differentiated area in 

 the roof has reached its maximal differentiation. A section from an embryo of 

 this size (embryo No. 409) is reproduced to show the distortion and its influence upon 

 the topography of the area membranacea. The two photomicrographs (figs. 62 and 

 63) show the extreme collapse and distortion of the roof of the fourth ventricle. In 

 the figure of higher power (No. 63) the membranous area appears facing poste- 

 riorly, due to the shrinkage; the proper leader runs to this area. It shows the differ- 

 entiation from the adjoining typical ependyma which is characteristic of the fully 

 developed area membranacea superior. 



In a beautifully preserved and sectioned human embryo of 21 mm. (No. 460) 

 in the collection of the Carnegie Institution of Washington the area membranacea 

 superior appears as a sharply delimited area (figs. 64 and 65). These figures give 

 a very good idea of the definiteness of the area when the fixation and dehydration 

 approach the perfect. The tissue of this membranous area lining the ventricle 

 here appears to be wholly lacking in an epithelial covering; the mesenchyme seems 

 to serve as the ependymal lining. Study of this area, however, through different 

 stages argues most strongly against such a view. 



